1. Field of the Invention
The present invention relates to an electric power system for a vehicle equipped with a plurality of electric power storage devices such as rechargeable batteries capable of supplying different terminal voltages to various types of electrical components as electrical loads mounted to a vehicle.
2. Description of the Related Art
There is known an electric power system for a vehicle. The electric power system is equipped with a plurality of electric power storage devices such as a main battery and a sub battery which are rechargeable. Each of the electric power storage devices outputs a different terminal voltage. Such an electric power system is comprised of an alternator, various electrical loads, and electric power storage devices such as a main battery and a sub battery. The alternator is driven by a rotary power transmitted from an internal combustion engine and controlled to generate en electric power of a specified target voltage. The alternator supplies the generated electric power to the main battery and various types of electrical loads.
The sub battery is electrically connected to the main battery through an electric power converter for a bi-directional transmission in order to transmit the electric power between them in bi-directions. Hereinafter, this type of the electric power system for a vehicle will be referred to as the “dual battery type electric power system for a vehicle”.
The Japanese patent JP 3465293 as a conventional technique has disclosed such a dual battery type electric power system for a vehicle. JP 3465293 has proposed that a main battery is composed of an electric double-layer capacitor (EDLC) and is charged with a constant current supplied from an alternator, and the alternator also generates a regenerative electric power using a regenerative brake during a vehicle speed reduction or braking in addition to during forward driving of the vehicle. During a period of time when the vehicle is moving other than the vehicle speed reduction, the electric power stored in the sub battery is, in preference to the electric power stored in the main battery, supplied to the various electrical loads.
The dual battery type electric power system for a vehicle disclosed in JP 3465293 adopts a constant current regenerative method of charging the sub battery during a period of time of a vehicle speed reduction. The electric power (hereinafter, referred to as the “residual generated electric power”), which the alternator can supply to the sub battery through an electric power converter for a bi-directional transmission, is obtained by subtracting an electric power to be consumed by both the electrical loads and the main battery from the maximum generated electric power of the alternator. However, the magnitude of the residual generated electric power generally fluctuates according to various operational conditions of the alternator, the electric loads, and the main battery.
For example, when a charging current for charging the sub battery is larger than a current of the residual generated electric power, the main battery discharges the electric power stored therein in order to compensate the difference of those electric power. As a result, the terminal voltage of the main battery is decreased.
On the other hand, when a charging current for charging the sub battery is smaller than a current of the residual generated electric power, it becomes difficult to obtain the effect of economy in power consumption by generating the regenerative electric power during a vehicle speed reduction or braking.
A large voltage drop of the main battery causes a terminal voltage drop to be supplied to the electrical loads which are electrically connected to the main battery in addition to wasteful discharging of the main battery. Thus, the voltage drop of the main battery will cause head lamps of a vehicle to decrease its intensity of illumination, and the speed of windshield wipers of the vehicle to be varied or drop. Because this gives the driver of the vehicle a sense of discomfort when driving, it must be avoided.